There is a significant demand for successful acceptance of transplanted organs into the compromised patient. T cells interfere with this process; therefore, new approaches to block their activity against the graft must be identified. Previous work suggested that pharmacological agents that uncouple signal transducer and activator of transcription (Stat)5a and StatSb activity can prolong allograft survival and Stat5a/b knockout mice can display graft tolerance. This project is based on the hypothesis that Stat5a/b regulates previously uncharacterized T-cell survival genes, both pro- and anti-apoptotic, and seeks to identify these novel genes to monitor their induction during transplantation tolerance. The central hypothesis is that inhibition of Stat5a/b can promote permanent acceptance of allografts without chronic therapy associated with current and toxic immunosuppressants. The objective of this pilot study is to identify murine Stat5a/b responsive genes as regulators of T-cell activity believing that these new targets should result in novel biomarkers that can be used in the design of new tolerance inducing regimes within the following interrelated aims: 1. Identify Stat5a/b target genes (StatS Dependent Gene Database S5GD) responsible for T-cell death from Stat5a/b-/- mice as compared to Stat5a/b+/+ mice using Affymetrix microarrays. 2. (A) Identify "Acceptor vs. Rejecter" Gene Database (ARGD) from C3H heart transplanted Stat5a/b-/- mice vs. Stat5a/b+/+ mice on Day 7 using Affymetrix microarrays. (B) Identify Graft Specific Gene Database (GSGD)from C3H heart acceptor Stat5a/b-/- mice on Day 7 compared to Day 50 using Affymetrix microarrays. 3. Identify the overlapping biomarker gene databases (by aligning S5GD, ARGD and GSGD) and validate putative T-cell regulatory genes in mice undergoing organ graft rejection compared to tolerized mice by Q RT PCR (heart rejection model). Relevance to public health: We propose to focus on two proteins critical for immune cells to cause rejection !of transplanted organs, namely StatSa and StatSb. We seek to understand the mechanism by which blocking these proteins can enhance the life of the transplanted organ. New strategies should result that will allow for therapeutic approaches to be designed. Additionally, novel biomarkers will be identified to more efficiently monitor a patient's response to a given therapy